Ultrasonic acoustic emissions from excised stems of two Thryptomene species

Stems of Thryptomene vaxicola (A. Cunn.) Shau. cut in air and immediately placed in water at 20°C showed ultrasonic acoustic emissions after 8 to 12 h. whereas no emissions were found in Thryptomene catycina Lindl.) Stapf, within 100 h. Stem water potential declined in a similar manner in both species: a high frequency of acoustic emissions stalrted at approximately –1 000 kPa in T.saxicola., but had not commenced at –5300 kPa in T. calycina. The rates of water uptake were similar in both species. Acoustic emissions in T.saxicola placed in water were prevented h recutting 15 cm of the stems under water. For comparison, cut stems which were air-dehydrated at 20°C exhibited a primary peak in the frequency of emission after 3–5 h in T. saxicola and 15 h in T. catycina, corresponding to a water potential of –1000 kPa and less than –5000 kPa. respectively. After 6. 16. and 24 h of dehydration in air and subsequent placement in water for 24 h. the rates of water uptake of T. calycina were not lower than in controls not held dry, How ever. in T. saxicola water uptake rates had decreased after 16 h of dehydration and were very low after 24 h. The results show that ultrasonic acoustic emissions may occur in freshly cut stems placed in water, and that they occur at a water potential similar to that in stems allowed to dehydrate in air. The number ol emissions in T.saxicola stems placed in water was apparently too low to inhibit water uptake. When held dry. the cavitated xylem elements may be a cause of reduced water uptake in T. saxicola.